In the hydrocarbon recovery industry, pumps are used at the lower ends of wells to pump water or oil to the surface through production tubing positioned within a well casing. The production tubing is generally positioned within a casing, with an annulus formed therebetween. Fluid from the formation enters the annulus and is pumped upwardly through the production tubing. Power is transmitted to the pump from the surface using a rod string positioned within the production tubing. Rod strings include both “reciprocating” types used with “beam pumping units”, which are axially stroked, and “rotating” types for use with progressive cavity pumps, which rotate to power progressing cavity pumps.
As to both reciprocating and rotating type pumps, if the rate of pumping exceeds the rate of supply by the formation, fluid level in the annulus will be lowered. If the fluid level drops too low, and especially if the fluid level falls below the upper end of the pump, the pump can be damaged. Likewise, if the rate of supply by the formation exceeds the rate of pumping, fluid level will rise. If the fluid level is too high, however, the well is not producing at maximum capacity, and production revenues are not maximized. There is accordingly a trade-off between pumping at high and low fluid levels.
Some systems have been proposed for timing pump strokes of a reciprocating type rod. U.S. Pat. No. 4,873,635 to Mills discloses a pump-off control device for use with a reciprocating type rod. The device uses the occurrence of a pump-off or partial pump-off condition, which means the fluid has dropped below full barrel. As the fluid level drops to below full barrel, the downstroke naturally speeds up due to the presence of air. The device measures the length of time required for the pump to downstroke successive numbers of times, and when the time differential reaches a predetermined value indicating a pump-off condition, the well is shut in for a time interval.
U.S. Pat. No. 4,490,094 discloses a method whereby instantaneous speeds of revolution for a beam pumping unit prime mover rotor are compared to predetermined values to correct pumping unit operation, such as during pump-off, mechanical malfunction, electrical operating inefficiency, or pumping unit imbalance. These systems are limited to use with reciprocating type pumps.
Particularly as to progressive cavity pumps coupled with rotating rod strings, as fluid level in the annulus drops, the hydrostatic pressure is reduced and the prime mover that powers rotation of the rod must work harder. Conversely, a higher fluid level increases hydrostatic pressure, which assists a progressive cavity pump by reducing the “head,” which is a spacing between the fluid level and the surface. Production from the well can be optimized if the fluid level is maintained at a certain value or range of values. The prior art discloses a number of approaches to detecting fluid level. For example, U.S. Pat. No. 6,085,836 discloses a method of transmitting sonic signals into the annulus to determine fluid level. U.S. Pat. No. 5,372,482 discloses a way to monitor fluid level indirectly from variation in the power consumption of an electrical motor. This patent eliminates the need for downhole pressure sensors and amperage monitors.
In recent years, gas producing companies have discovered that gas can be profitably produced by drilling into coal beads and pumping out the water. Lowering the hydrostatic head pressure by removing the water permits the gas to flow to the surface. The progressive cavity pump has been found to be a very cost effective way to remove the water from these coal sands and to lower hydrostatic head pressure. The fluid level in the annulus above the progressive cavity pump needs to be controlled at a level that always gives sufficient pump submergence. If there is insufficient pump submergence the progressive cavity pump can be damaged or destroyed, which is expensive to repair or replace.
Other patents of interest include U.S. Pat. Nos. 6,456,201; 6,481,499; 6,554,066; and 5,291,777.